As computer speeds are increased, it becomes necessary to package circuitry at increasingly greater densities. One method for achieving this is to put the active circuit chips in smaller packages and position these packages as close together as possible on a printed circuit board. As this is done however, heat densities rise, sometimes beyond the cooling capabilities of forced air convection. It then becomes necessary to resort to cold plates or other heat removing devices.
One way to achieve good cooling of high density packaging is to place a liquid cooled, cold plate in direct contact with the circuit package. To do so, however, is difficult due to variations in dimensional tolerances and in the relative heights and shapes of the circuit components on a printed circuit board. This problem may be alleviated through the use of a compliant interface between the cold plate and the circuit components. Such an interface must be forgiving of any variations or irregularities; it must be deformable so as to comply with the surfaces of the individual circuit packages. It is also desirable that the interface be made of a material that is heat conductive and electrically insulative, since the contact resistance decreases as a more intimate contact is made between the cold plate and the heat generating circuit components and to prevent electrical shorts between the cold plate and circuit components.
Many different resilient materials and structures have been tried in attempts to achieve a compliant interface. Some of those suggested have been a mat consisting of rubber tubing, U.S. Pat. No. 3,833,836; a cellular or honeycomb structure, U.S. Pat. No. 3,302,067; a multi-parallel leaf padding means, U.S. Pat. No. 3,212,564; wire mesh strips, U.S. Pat. No. 3,267,333; an enclosure for liquid immersion, U.S. Pat. Nos. 3,851,221 and 3,124,720; and a gelled cooling cushion, U.S. Pat. No. 3,802,220.
A paste and film interface as disclosed herein has been found to provide a more compliant interface for packaging electric circuit components than the previously used resilient structures. The circuitry is also more accessible and the coolant is more economical than previously used flurocarbons. Tests have shown the paste mixture used herein to possess good heat transfer characteristics and the film used to contain the paste also provides electrical insulation between the interface and the components, since the paste contains metal particles. In combination the paste and film provide a better mating between the cold plate and the circuit components of a variety of shapes and different heights, and thereby provides better heat transfer over an increased contact surface area.